Our goal is to determine if the activities of the cytosol and mitochondria of mammary gland are coordinated and how this coordination is regulated during transformations in cellular function. Mammary gland was chosen as a model because this tissue proceeds through distinct, hormonally-controlled, stages. We have established that there is extensive transport of compounds back and forth across the mitochondrial membrane in mammary gland. This exchange acts as a means of communication between the cytosol and the mitochondrial matrix. Our studies have revealed that changes in this exchange accompany alterations in cellular function and are, therefore, under hormonal regulation. Glands from pregnant animals lack at least two specific transport systems that are essential to the function of the lactating gland. Furthermore, differences have been uncovered between the glands from pregnant and those from lactating animals in their utilization of substrates by pathways involving an interaction of both compartments. After the differences between the glands from pregnant and lactating states have been fully exploited, the role of individual and specific hormones on the regulation of interactions between intracellular compartments will be examined.